Billie Eilish has several middle names. Eilish is in fact one of them. One of them, however, is “Pirate”. Because her parents allowed her big brother to choose one of them when she was born.

Why she doesn’t go by Pirate, I’ll never understand.

I was thinking more when you become an adult you pick your name, I did that and over a decade later I still feel like my name suits me.

It's a normal name, nothing crazy or spelled weird.